The present invention relates generally to testing and sampling operations performed in subterranean wells and, in an embodiment described herein, more particularly provides an annulus pressure operated downhole choke and associated methods.
In a conventional fluid sampling operation performed for a subterranean well, a sample chamber is attached to a tubing string and positioned within the well in order to take an in situ sample of the fluid flowing through the tubing string. Preferably, the sample is taken in relatively close proximity to a formation from which the fluid originates. Additionally, it is generally desired to take the sample in steady state flow conditions and at a fluid pressure greater than the bubble point of any oil in the sample.
To achieve the desired fluid pressure at the downhole sample chamber while the fluid is flowing through the tubing string, a choke is typically installed at the earth's surface and connected to the tubing string to restrict fluid flow through the tubing string at the earth's surface. However, due to the usually great distance between the choke and the formation and resulting wellbore storage effects, the desired steady state flow is not established until a substantial amount of time after flow through the choke is commenced. If a sample is taken during this long period of unsteady flow, the sample may include proportions of oil and gas which are uncharacteristic of the formation fluid and, therefore, impair any analysis of the formation relating, for example, to optimum rates of production from the formation, etc.
Furthermore, it is at times helpful to take additional samples at differing downhole fluid pressures, differing flow rates, etc., in order to more accurately analyze the formation, predict the optimum rate of production, etc. In these situations a corresponding additional choke having a different flow restriction is installed at the earth's surface prior to taking each of the additional samples. Unfortunately, each time an additional choke is installed, a substantial period of time must again elapse before steady state flow conditions are established.
The expense of performing these operations could be significantly reduced if an apparatus and/or method were developed to minimize or eliminate the time period spent waiting for flow conditions to stabilize at the sample chamber. Thus, from the foregoing, it can be seen that it would be quite desirable to provide a choke which may be installed in the tubing string in close proximity to the sample chamber, thereby substantially eliminating the effect of wellbore storage on fluid flow through the choke. In addition, it would be desirable to control the downhole choke using fluid pressure applied to the annulus at the earth's surface, and to alternately provide substantially unrestricted flow and restricted flow through the choke. It would also be desirable to provide methods whereby a downhole choke may be operated by application of annulus pressure, and methods whereby multiple downhole chokes and multiple sample chambers may be installed in the well to enhance analysis of the formation. It is accordingly an object of the present invention to provide such a downhole choke and associated methods of using same.